Wrongly expect "modified" key in content.json

Hello,

I moved my zite on a new server, and I caught a bug in ZeroNet in the process.

So I already have a minimal `content.json` setting file, which among other things specify that `*.pdf` files are optional (I have a lot of that on my website and it would explode the 10MB limit of ZeroNet without this setting).

But I couldn't `siteSign` because ZeroNet was expecting the `"modified"` key to be there :

Site:1PaBLo..Mhif Content.json parse error: KeyError: 'modified' in ContentManager.py line 213 > ContentDbDict.py line 61 > ContentDbPlugin.py line 198 > ContentDb.py line 85
Site:1PaBLo..Mhif File content.json not exist yet, loading default values...

Because of that it ignored my existing `content.json` file and thus refused to sign the Zite, since it was way too big with the PDFs.

The ZeroNet documentation says that `"modified"` is automatically generated, so I think that is a bug and not expected behavior.

For now the fix for users in my situation is to put `"modified": 0` for example in the `content.json` file. Then ZeroNet accepts the existing `content.json` file and everything works as usual (i.e., do a `siteSign`, then a `sitePublish`, then visit the zite locally, and voilà).